1. Field of the Invention
The present invention relates to a light guide, an illumination device, and an image reading apparatus. The present invention is suitably applied particularly to an image reading apparatus such as an image scanner, a copying machine, facsimile machine or the like that reads an image while illuminating the surface of an original.
2. Description of the Related Art
A need for reduction in cost and increase in the operation speed of image reading apparatuses has been growing. In response to this, the sensor size has been made smaller, necessitating decreases in the reduction ratio of reduction optical systems. Consequently, more bright original illumination systems are demanded in order to achieve image qualities as high as before. To this end, light emitting diodes (LED) having high light emitting efficiency may be used.
When point light sources like LEDs are used in the light source, a plurality of point light sources are arranged along the main scanning direction in many cases. If light beams emitted from the plurality of point light sources are directly delivered to the surface of an original, illumination unevenness with respect to the direction along which the light sources are arranged will occur, leading to density unevenness in the read image. The publication of Japanese Patent No. 4533235 and Japanese Patent Application Laid-Open No. 2008-123766 disclose methods of reducing illumination unevenness in the image reading apparatus using a plurality of point light sources. The publication of Japanese Patent No. 4533235 discloses an illumination apparatus that uses a light guide in order to reduce illumination unevenness with respect to the direction of arrangement of light sources. In this apparatus, direct light beams and light beams reflected in the light guide are aligned. Japanese Patent Application Laid-Open No. 2008-123766 discloses an illumination apparatus provided with a light guide with a light exit surface having a condensing function.
However, there is a need for image reading apparatuses having a further increased reading speed. The illumination apparatuses disclosed in the publication of Japanese Patent No. 4533235 and Japanese Patent Application Laid-Open No. 2008-123766 do not meet this need because of insufficient condensing efficiency with respect of the sub-scanning direction (i.e. the direction perpendicular to the main scanning direction or the direction of arrangement of the light sources). Specifically, in the apparatus disclosed in the publication of Japanese Patent No. 4533235, while it is possible to align the direct light and the light having been reflected multiple times in the light guide, condensing of the direct light and the multi-reflected light is insufficient in particular with respect to the sub-scanning direction.
In the apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-123766, direct light and multi-reflected light having passed through the light guide are condensed only by the light exit part. Therefore, it is not easy to achieve both condensing of each light beam and condensing of direct light and multi-reflected light. For this reason, the condensing efficiency with respect to the sub-scanning direction is insufficient.